A production process of a conventional ridge waveguide type semiconductor laser device and a sectional view of the structure thereof is shown in FIGS. 2(a) to 2(c).
In the figure, 201 designates a substrate, 202 designates a first conductive type clad layer, 203 designates an active layer, 204 designates a second conductive type clad layer, 205 designates a second conductive type contact layer, 206 designates a protective film which is made of an insulator, 207 designates an electrode of the epitaxial side, and 208 designates an electrode of the substrate side.
A method of producing the conventional ridge waveguide type semiconductor laser device is as follows. First, a double heterostructure (hereinafter referred to as "DH") in which the active layer 203 is sandwiched from upside and downside between the two clad layers is produced (FIG. 2(a)). A ridge portion 209 is formed by an etching process using a stripe-like etching mask. During this process, in non-ridge portions 210, etching is performed to a middle level of the second conductive type clad layer 204 which is provided on the active layer 203 (FIG. 2(b)). Thereafter, the side surfaces of the ridge and the surfaces of the non-ridge portions are covered by the protective film 206, thereby preventing a current from flowing through a portion other than the top of the ridge. The electrode 207 is formed on the protective film and the upper portion of the ridge, and the electrode 208 is also formed on the substrate side.
According to this structure, a current is injected through the ridge portion of the clad layer, and then into the active layer 203. Therefore, the current is concentrated into the region of the active layer 203 which is under the ridge portion, so that light having a wavelength corresponding to the band gap of the active layer 203 is generated. Generally, the band gap of the active layer is smaller than those of the upper and lower clad layers, and the refractive index of the active layer is larger than those of the upper and lower clad layers. Consequently, carriers and light can be effectively confined in the active layer, and the threshold current for laser emission can be lowered.
In contrast, in the non-ridge portions 210, the protective film 206 which is smaller in refractive index than the semiconductor portions is formed, and hence the effective refractive index of the portions of the active layer 203 which are under the non-ridge portions 210 is smaller than that of the portion which is under the ridge portion 209. As a result, the generated light is confined in the active layer 203 which is under the ridge portion 209.
Because the electrode 207 is intended to contact only the contact layer which is at the top of the ridge portion, the conventional ridge waveguide type stripe semiconductor laser device is produced in the following manner. The surface of the epitaxial side is covered by the protective film 206. Thereafter, a stripe-like window is opened in the resist by a patterning process using the photolithography, and only a part of the protective film 206 which is above the ridge portion 209 is etched away, thereby exposing the contact layer 205. In some case, a protective film made of SiN.sub.x or the like is formed on the side walls of the ridge.
In a production of a laser device of a single transverse mode, usually, a very accurate positioning technique is required, because the width of the upper portion of the ridge is about several microns at maximum, and it is difficult to use in this process a process simplifying technique such as a self alignment. Such a photolithography technique which is complex and fine complicates the device production steps and lowers the device production yield.
When an SiN.sub.x film is formed on side walls of a ridge, a depletion layer (about 0.1 .mu.m) is formed in the side faces of the ridge on the side of the surface, This produces a problem in that the effective width of the current channel is reduced and the pass resistance is increased.
In a conventional ridge waveguide type stripe semiconductor light-emitting device, moreover, the ridge portion is formed by an etching process, and hence it is difficult to set the thickness of an etched clad layer in the non-ridge portion on the active layer, and the width of the lower portion of the ridge to have a uniform value. As a result, even a very small difference of the thickness of the clad layer in the non-ridge portion causes the effective refractive index of the active layer in this portion to be largely varied, or the lateral width of the current injection region to be largely varied. Consequently, it is difficult to produce a semiconductor laser device in which the threshold current is low and the beam divergence has a desired constant value, at a high device yield and with excellent reproducibility.
In order to solve the problem regarding a thickness, a method is proposed in which the thickness of a clad layer above an active layer is accurately controlled by using the crystal growth rate upon a crystal growing process, a protective film made of an insulator is formed in an area other than a ridge portion, and the ridge portion is regrown (for example, JP-A-5-121822 (The term "JP-A" means a published unexamined Japanese application)). The production process and the structure of such a laser device is shown in FIGS. 3(a) to 3(c). The structure and the production method therefor is explained below. In the method, steps of forming a double heterostructure composed of a first conductive type clad layer 302, an active layer 303, and a second conductive type first clad layer 304 (FIG. 3(a)) are conducted in the same manner as those of the above described conventional technique. The method is characterized in that a second conductive type second clad layer 307 and a contact layer 308 are then selectively grown only in a stripe region 306 to a ridge-like shape by using a protective film 305 (FIG. 3(b)). The side faces and both sides of the ridge composed of the second conductive typo second clad layer 307 and the contact layer 308 are covered by a protective layer 310. Electrodes 309 are then formed on the upper and lower sides of the resulting product, respectively.
In this way, the ridge portion is formed by a method which does not use an etching process. Therefore, the thickness of the clad layer (304) which is above the active layer in the non-ridge portions can be accurately controlled, and the control of the effective refractive index can be easily performed.
In the above-described ridge waveguide structure, however, light leaks toward the electrode, in the vicinity of the boundary between the bottom of the ridge and the protective film. Because of light absorption and reflection due to the electrode metal, it is difficult to control the light distribution in the waveguide.
Moreover, when a light waveguide structure is to be formed in order to attain a single transverse mode, the ridge width of the top of the ridge must be set to be about 1 .mu.m. In other words, the contact area between the contact layer and the electrode is very small, and hence the contact resistance between the contact layer and the electrode is increased. Furthermore, the oxidation of the surface of the clad layer on the side walls of the ridge causes degradation of the laser characteristics, reduction of the reliability, and the like. As a result, the laser characteristics of semiconductor light-emitting devices are varied and it is difficult to improve the product yield.